Direct injection internal combustion engines, in which fuel is injected directly into the combustion chambers of the internal combustion engine, are known in the art. A direct injection internal combustion engine may be operated in different operating modes, for instance, in a stratified operation or a homogeneous operation. Stratified operation is used especially for small loads, whereas homogeneous operation comes into use for greater loads on the internal combustion engine.
In stratified operation, the fuel is injected into the combustion chamber during a compression stroke of the internal combustion engine in such a manner that, at the time of ignition, there is a fuel cloud in the immediate surroundings of a spark plug. This injection may proceed in different manners. Thus, it is possible that the injected fuel cloud is at the spark plug already during, or immediately after the injection, and is ignited by it. It is also possible that the injected fuel cloud is supplied to the spark plug by a charge movement, and is only then ignited. In the case of both combustion methods there is no uniform fuel distribution, but rather a stratified charge.
Stratified operation may provide that, using a very slight quantity of fuel, the smaller loads present are able to be performed by the internal combustion engine. Greater loads, however, may not be handled by stratified operation.
In the homogeneous operation provided for such greater loads, the fuel is injected during an intake stroke of the internal combustion engine, so that a turbulence and thereby a distribution of the fuel is still able to occur in the combustion chamber right away before the ignition of the fuel/air mixture. To that extent, homogeneous operation corresponds approximately to the operating manner of internal combustion engines in which, in the usual manner, fuel is injected into the intake pipe. If necessary, one may also switch over to homogeneous operation for smaller loads.
In stratified operation, the throttle valve in the intake pipe leading to the combustion chamber is opened wide, and combustion is controlled and/or regulated only by the fuel quantity to be injected. In homogeneous operation, the throttle valve is opened and closed as a function of the required torque, and the fuel quantity to be injected is controlled and/or regulated as a function of the aspirated air mass. In both operating modes, that is, in stratified operation and in homogeneous operation, the fuel quantity to be injected is also controlled and/or regulated as a function of a multitude of additional operating variables towards an optimal value with respect to fuel savings, exhaust gas reduction and the like. In this context, the control and/or regulation is different for the two operating modes.
Also known in the art are internal combustion engines, in which fuel is injected into the combustion chambers of the internal combustion engine, not by a single injection, but subdivided into several consecutive injections, especially by a main injection and a subsequent postinjection. For example, an internal combustion engine is described in published European Patent Application No. 0 971 104, in which the fuel is injected into the combustion chambers by a postinjection during an exhaust stroke of the internal combustion engine, that is, clearly after the ignition time of a fuel/air mixture. The fuel quantity injected by the postinjection arrives unburned in a catalytic converter of the internal combustion engine, and is ignited there. This leads to heating of the catalytic converter, so that temperatures required for the regeneration of the catalytic converter may be achieved in the catalytic converter.
Internal combustion engines having direct fuel injection, in partial-load operation may not be driven steadily in thermodynamically optimal, throttle-free, quality-controlled stratified operation. In order, for example, to ensure an effective fuel tank venting or an efficient regeneration of a nitrogen oxide (NOx) catalytic converter, at certain time intervals (depending on the engine speed and the load state of internal combustion engine), a throttling of the intake air of internal combustion engine and a stoichiometric (ë=1) or substoichiometric (ë<1) fuel/air ratio connected therewith has to be set. As soon as the air intake supply is throttled, the fuel has to be injected during the intake stroke and not—as in the (throttle-free) stratified charge operation—during the compression stroke. The internal combustion engine then behaves, with respect to fuel usage and exhaust gas emission, like a conventional internal combustion engine having manifold injection.
It is known from other systems that, during the special functions described above, fuel tank venting, catalytic converter regeneration or others, one may inject the fuel, within the scope of a one-time injection in the intake stroke. Displacing the injection time into the compression stroke, in conjunction with extreme air intake throttling, causes very great soot emission, and is therefore not used in practice.
However, when the internal combustion engine is operated using extreme intake air throttling and stoichiometric mixture composition, the spray orifices of the injectors coke up, with the effect that, after a certain running time of the internal combustion engine the injection time has to be permanently increased, so that, at a predefined load, the fuel supply may be held constant. By increasing the injection time tq it is achieved that the time cross section ((dm/dt)*tq) for the fuel supply per work cycle remains unchanged.
Although the special functions for fuel tank venting and catalytic converter regeneration are only activated for a short time, it is ensured that the coking up of an injector valve, that is, a spray orifice internal coking creating flow interference, has to be avoided in this operating phase under all circumstances. Otherwise the injection time has to be increased above the limits of the application, after a longer operating duration.
Therefore the present invention effectively avoids injector orifice internal coking of the spray orifices of a fuel injector, especially at high air intake throttling and stoichiometric or substoichiometric mixture composition.
In order to achieve this, the present invention provides that the predominant proportion of the entire fuel quantity is injected by the main injection, and a change in the torque developed by the internal combustion engine is controlled and/or regulated exclusively via a change in the injection time during the main injection, and the fuel is injected by the main injection and the subsequent postinjection before the ignition time of a fuel/air mixture.